Optical device for determining the location of a reflective target

ABSTRACT

The invention relates to an optical device by means of which the direction of a graticule may be measured with high precision. To this end, the graticule is illuminated by a spotlight and the light reflecting from it collected by a theodolite telescope and a locating detector. To increase the range and to improve accuracy of measurement, use is made not of a conventional spherical metal graticule but a sphere of transparent material the refractive index n of which is greater than the square root of 2 and smaller than 2.

BACKGROUND

The invention relates to an optical device for determining the locationof a reflective target, having a spotlight for illuminating the target,an imaging system for imaging the target onto a position-resolvingdetector device and an evaluation device for determining the positioncoordinates of the target image.

U.S. Pat. No. 3,188,739 discloses a target which is a transparent spherehaving a marked center. The marking is carried out by means of adrilling along a radial line of the sphere as far as the center of thesphere. In addition, the marking of the center of the sphere can bereinforced by illuminating the drilled hole. Another possibility formarking results from the use of luminescent materials which are meltedinto the center of the sphere. Using spheres marked in this way, adetermination of the distance between two points in space is possible ina known way by means of triangulation with the aid of theodolites.

German Publication DE 40 13 576 Al discloses an opaque glass spherewhich is used as a target for determining measurement points usingoptical sights. As a result of the translucent, light-scatteringmaterial of the sphere, a homogeneous lightness of the sphere surface isachieved, which makes sighting from a large angular range possible.

U.S. Pat. No. 5,207,003 describes a target having at least fourconventional retroreflectors and a spherical reflector, with which thethree-dimensional position and the angular location of the target ismeasured. For this purpose, the target is illuminated by a light sourcevia a semitransparent scattering mirror. The light reflected from thespherical reflector and the four retroreflectors passes via some opticsto a photographic detector. In this case, the center of curvature of thespherical reflector and the retroreflectors are imaged as spots oflight. From the coordinates of these spots of light on the detector, thethree-dimensional position and the angular location of the target andthe object connected to it are determined by computation.

Further optical devices of the said type are known and extensivelyrepresented in the publication Technische Rundschau No. 39, 1988, pp.14-18 under the title "Theodolitsysteme fur industrielle und geodatischeMessungen" Theodolite systems for industrial and geodatic measurements!by W. Huep, O. Katowski, and in the publication Opticus No. 1/94, pp.8-9 from Leica AG in an article under the title "Theodolite-Sensoren furdie industrielle Vermessung" Theodolites-sensors for industrialsurveying!.

The theodolite systems which are described in these publications areemployed for the non-contacting surveying of surfaces such as, forexample, the panels of aircraft or car body parts. The theodolitesystems used have a compact structure with a theodolite telescope as animaging system, a spotlight arranged coaxially therewith forilluminating the target, a video camera with a CCD array and anelectronic evaluation device, which may also contain an image processingcomputer and appropriate software. In addition, the theodolite systemmay also be equipped with a distance measuring device.

The theodolite systems are equipped with electronic circular taps andmotorized drives for the horizontal and vertical adjustments of thetelescope, with the result that an automatic, remotely controlledmeasurement sequence is possible. The horizontal and vertical anglesindicate the directions to the targets in a given coordinate system.

In order to achieve an objective and precise measurement of direction,the video camera is used with the inclusion of image processingalgorithms. The target image produced on the CCD array of the camera isevaluated with the aid of suitable algorithms and the center of thetarget image is determined. The coordinates of this center on the CCDarray are then converted by means of software into displacement angleswhich, together with electronic circle read-outs yield the direction tothe target.

Using the measurements of direction, the three-dimensional targetcoordinates are in turn determined if there is additional information.In the case of the polar method, a distance measurement is undertaken inaddition to the angular measurement. On the other hand, thetriangulation method uses a second theodolite system of known positionand determines the positions of the targets trigonometrically via pureangular measurements.

A light-emitting diode (so-called high radiance diode) emitting withhigh beam density in the near infrared is generally used as a radiationsource for the spotlight.

Used as targets for surveying the object are surface-reflective spheres,for example chrome-plated polished steel spheres which exhibit therequired property that the same target image is presented independent ofthe direction of observation of the respective theodolite. Located onthese surface-reflective spheres are centered fastening elements, by useof which these targets can be fitted reproducibly at the positionsprovided therefor on the surface to be surveyed. Rod-shaped mounts canalso be used, on which the spheres rest.

The reflective spheres used, which normally have a diameter of about 12mm, generate a virtual image, apparently located in the interior of thesphere, of the spotlight pupil of the theodolite approximately in thefocal plane of the sphere surface, that is to say at approximately onequarter of the sphere diameter behind the vertex of the sphere surface.This image is observed using the telescope of the theodolite or isrecorded by video camera and displayed on the CCD array.

Because of the small focal length of the surface-reflective spheres, ofabout 3 mm in the case of sphere diameters of about 12 mm, the pupilimage in the sphere is already small, limited by diffraction, aboveabout 1.5 m distance and is hence smaller than a receiving element ofthe CCD array. In order that an evaluation can nevertheless be carriedout, the theodolite telescope has to be defocused in such a way that thetarget image is distributed onto several pixels of the camera. Thecenter of this spot of light, obtained in this manner, can be determinedby determining the center of gravity or by evaluating the contour.Nevertheless, the different intensity distribution of the radiation inthe spot of light and its unsharp edge leads to a measurement of thedirection of the sphere which is only of limited precision. Likewise,slight contamination of the sphere leads to a different intensitydistribution of the radiation in the spot of light and hence tomeasurement errors.

A further problem of the imaging by means of reflective sphericalsurfaces results from the fact that the radiation power reflected backby these spheres into the telescope objective falls off with the 4thpower of the distance from the spotlight limiting distance. This has theconsequence that this measurement method can be applied only up todistances of about 10 m. In the case of greater distances, the receivedsignal is no longer sufficient for a measurement of the direction andhence ultimately for a three-dimensional position determination, even iflarger spheres are used.

SUMMARY OF THE INVENTION

Proceeding from this prior art, the invention is based on the object ofspecifying an optical device and a method for determining the locationof the target image on a position-sensitive detector, in which thisdetermination of location and hence the measurement of the direction ofthe target becomes possible over relatively large distances, can becarried out with higher precision, is insensitive to contamination andis insensitive to imaging conditions of the target which are limited byfastening elements.

According to the invention, this object is achieved by employing asphere as a target which is transparent to the spotlight illuminationand has a refractive index n between √2 wherein 2, and the target imageis at least part of a circular annular area, the center of which,determined by image evaluation, indicates the location of the target. Aparticular method comprises the steps that the imaging system isdefocused and the multiple circular annular areas produced thereby arepicked up by the detector device and evaluated using mathematical meansto determine the center of the target.

Advantageous developments and improvements of the invention emerge fromthe features described below.

The advantages of the invention lie, on the one hand, in the dramaticincrease in the radiation power at the location of the detector deviceby means of the use of spherical targets which are transparent to theillumination radiation of the spotlight, whose refractive index for thisradiation is greater than √2 and smaller than 2. Spheres of this typehave the optical property that the major portion of the incidentradiation enters into the sphere material as a result of opticalrefraction and, after passing through the sphere, a specific proportionof the radiation is reflected at the rear surface of the sphere, that isto say at the interface between sphere material and external medium(including air). The major portion of this reflected radiation emergesfrom the sphere, with refraction, following a further passage throughthe sphere.

If one considers a parallel beam which is incident on the sphere, theindividual rays of this beam will pass through the sphere in the mannerjust described and, depending on the location of their entry into thesphere, these rays will leave with different exit angles and also atdifferent exit locations. The invention, then, makes use of theknowledge that rays within one area of exit locations, that is to saywithin a specific spherical zone, leave the sphere under such exitangles that they can be picked up by an imaging system. For instance,the telescope objective of a theodolite is envisaged as an imagingsystem. The radiation power which is reflected back into the telescopeobjective from such a spherical zone in this case falls off only withthe 3rd power of the distance.

A sphere having the refractive index n=√2 represents a limiting case. Amarginal ray which is incident tangentially on the sphere surface andenters into the sphere with refraction will be partly reflected at therear of the sphere and emerges tangentially on the opposite side of thesphere, to be precise parallel to the incident ray. It is registered bythe imaging optics of the theodolite, as are rays close to the axis,which run close to the axis of the theodolite and the center of thesphere. All other incident rays are reflected diver-gently in such a waythat they cannot pass into the telescope objective.

In the case of a refractive index which is only slightly greater than√2, there is already a corresponding spherical zone from which the rayspass into the telescope objective and by means of which the detectedsignal still only falls off with the third power of the distance. Thewidth and the diameter of the spherical zone are different, depending onthe refractive index of the sphere. With increasing refractive index,the diameter of this spherical zone decreases, being identical to thesphere diameter in the case n=√2, until it becomes zero in the otherlimiting case at a refractive index of n=√2. Outside the refractiveindex range mentioned, this spherical zone does not exist.

The following example will show how extraordinarily effective theincrease in the received power is when such a transparent sphere isused, a video theodolite having a telescope aperture of 42 mm diameterhaving been used:

Using a commercially available sphere made of glass with a refractiveindex of n=1.83 and having a diameter of 2 mm, at a distance of 10 m theradiation power received is higher by a factor of 50 than in the case ofusing a chrome-plated steel sphere having a diameter of 10 mm.

At a distance of 20 meters, the improvement factor is already 100,although the reflection coefficient after the passage through thetransparent sphere is only about 8% in comparison to about 60% in thecase of the chrome-plated steel sphere.

The intensity of the reflected radiation, which is increased by suchfactors, now makes it possible to survey the object at a correspondinglygreater distance from the theodolite.

A second advantage of the invention emerges from the fact that thespherical zone from which the radiation received by the theodolitetelescope emerges has the shape of a ring. This annular exit zone islocated coaxially in relation to the axis of the center of the sphereand the center of the exit pupil of the spotlight illumination and, byfocusing the theodolite telescope, is imaged in sharply drawn fashion inthe image plane, that is to say at the location of the detection device.Hence, when a video camera is used, the target image always illuminatesa large number of pixels of the CCD array, just as in the case offocused telescope adjustment, in contrast with the target image of asteel sphere. In addition, the annular radiation distribution nowadvantageously makes possible the use of circle-fitting algorithms withwhich the center of the sphere of the target and hence, by means of acombination with a distance measurement, its position in 3-dimensionalspace can be determined highly precisely. The precision of such anevaluation is considerably higher than the determination of the centerof an unsharp spot of light by determining the center of gravity orevaluating the contour, as already described at the beginning in thecase of the conventional methods.

The target according to the invention supplies a target image which canbe evaluated very well not only in the case of a focused setting of thetheodolite telescope but also in the case of a defocused setting. If thereflection of the illumination radiation takes place via a small mirroron the optical axis within the theodolite, then in the case of adefocused theodolite telescope, two coaxial, annularly radiating regionsoccur, which are brought about as a result of the fact that thereceiving objective is shadowed in its central region by the smallmirror. These annular zones can also be evaluated using mathematicalmeans for the purpose of determining the center of the target, theaccuracy of the determination of the center being able to be increasedfurther by means of averaging.

The annular radiation distribution and the use made possible thereby ofcircle-fitting algorithms offer a further great advantage. In manyinvestigations it has transpired that the location of the center of thesphere of a target can be determined with constantly high precision,even in the case of a dirty sphere surface or in the case of vignettingwhich is caused by the sphere mount. This effect results from themanifold redundant information of the circular annular area, so thatlocal interruptions to or blurring of the circular ring can be averagedout or removed from the evaluation. On the other hand, in the case ofusing metal spheres with defocused reflections, there is always the riskthat measurement errors which may not be eliminated occur as a result ofcontaminated surfaces.

As a rule, use is made of surface-polished spheres, since otherwise thescattered light which is produced on a rough surface would reduce theusable radiation intensity. Likewise, reflection-enhancing coatings canincrease the radiation intensity reflected by the sphere. The coatingcan be carried out over the entire sphere surface, since the proportionsof the radiation reflected at the transition into media of differentrefractive indices only lie in the percentage range here. For thisreason, the radiation intensity reflected by the sphere into thetheodolite telescope is primarily determined by the reflection at the"rear" of the sphere.

In this region, the reflection can be sharply increased by silvering asmall area. However, this advantage is obtained at the cost that thesphere has a preferential direction as a result of the partial silveringand cannot simply be placed on a holder without being oriented. Whenusing the triangulation method, in particular, in which two or moretheodolites are used for measuring the direction of the sphere astarget, the usable angular range for the theodolites is restricted bythe size of the reflective surface. Without silvering, the sphere may besighted from all sides.

An increase in the radiation intensity reflected by the sphere can alsobe achieved by means of different materials within the sphere. In thecase of using materials having different, stepped or continuouslyextending refractive indices, the rays are no longer propagated linearlywithin the sphere but on curved paths. As a result, the exit angles ofthe rays are influenced, so that in the case of a suitable refractiveindex profile, an increased proportion of the radiation can be receivedby the theodolite.

BRIEF DESCRIPTION OF THE DRAWINGS

Two exemplary embodiments of the invention will be explained in moredetail below by reference to the drawings, in which

FIG. 1 shows a schematic representation of the subject matter of theinvention having external target illumination,

FIG. 2 shows a schematic representation of the subject matter of theinvention with target illumination integrated in the theodolite,

FIG. 3 shows a schematic representation of the course of the ray througha transparent sphere,

FIG. 4 shows a camera image of the transparent sphere which is beingilluminated by the coaxially arranged spotlight of the video theodolite.

DETAIL DESCRIPTION OF EXEMPLARY EMBODIMENTS

Shown schematically in FIG. 1 is an optical arrangement having aspotlight 1 which, via a beam-splitting mirror 20, illuminates aspherical target 2 coaxially with respect to the optical axis 15, havinga rod-shaped mount 4 as support for the target 2, an imaging system 5with a focusing device 6, a position-resolving detector device 7, anevaluation device 9, an image processing computer 10 and a distancemeasuring device 11. The size relationships shown in FIG. 1 are not trueto scale.

The optical beam path is shown for two selected, mutually parallel raysB and C. Drawn in here are only the courses of the rays which aredecisive for the invention and which, following entry into the sphere 2and reflection at its rear surface, emerge from the sphere 2 such thatthey are picked up by the imaging system 5. The material of the sphere 2is transparent to the illumination radiation of the spotlight 1, thevalue of the refractive index lying between √2 and 2. Under thiscondition, the two rays B and C bound a bundle of rays which is pickedup by the aperture of the imaging device 5 and can be imaged by thelatter onto the position-sensitive detector device 7. The circularannular area 8 on the detector device 7 is produced by the totality ofall bundles of rays which in relation to the bundles of rays B-C shownby way of example, are rotationally symmetrical to the optical axis 15.

To enhance the reflection in the rear part of the spherical target 2,partial silvering 3 may be used. If silvering is provided, the target 2can, of course, no longer be placed in any rotation onto the mount 4,but must be aligned with the silvered area 3, as FIG. 1 shows.

The function of the beam-splitting mirror 20 will be explained below. Ingeneral, beam-splitting mirrors have the property of reflecting aspecific part of the incident light and of transmitting the remainingother part. Drawn in FIG. 1 are only the rays coming from the spotlight1 and reflected at the beam-splitting mirror 20, as well as the rayscoming from the target 2 and transmitted through the beam-splittingmirror 20. The ray components which are complementary thereto and notdrawn in here are lost and do not contribute to the imaging of thetarget. The reflecting of the illumination radiation via thebeam-splitting mirror 20 therefore always means a certain light loss. Onthe other hand, however, a theodolite which has no integratedillumination can thereby be retrofitted with illumination in a simpleway. It is necessary only to fit the beam-splitting mirror 20 in frontof the objective of the theodolite telescope and for the targets to beilluminated coaxially using the spotlight 1, in order therewith toeffect and to make use of an imaging of the reflected spherical zone ofthe target in accordance with the invention.

FIG. 2 shows schematically an optical device like FIG. 1, but withtarget illumination integrated in the imaging system 5. A spotlight 30illuminates the target 2 via a mirror 31 which is arranged on theoptical axis 15 of the imaging system 5. The illumination is carried outlikewise coaxially to the optical axis 15. The mirror 31 shadows a smallcentral area of the imaging beam path, as a result of which some lightfor the imaging is lost. This arrangement corresponds to the theodolitesystem explained at the beginning.

Here, the target 2 has no partial silvering, so that it can beilluminated unrestrictedly from all sides, and the reflected rays alwayssupply the circular annular area 8 on the position-sensitive detector 7.The presentation of this target image independent of the direction ofillumination of a theodolite is especially suitable for the applicationof the triangulation method, with which the angles to the target 2 aredetermined from several theodolites.

FIG. 3 shows schematically the courses of some rays in a sectional planethrough the center of the sphere of the transparent target 2 having arefractive index between √2 and 2. For reasons of clarity, only theparallel rays A, B, C and D which are incident above the axis 15 aredrawn in. Depending on the distance from the axis 15, after passingthrough the sphere the rays emerge once more from the sphere finally atdifferent angles. The rays A and D are in this case reflected back atsuch exit angles that they pass by the optical imaging device 5 (FIG.2). Only those rays within the region which is hatched and bounded bythe rays B and C pass via the imaging device 5 onto theposition-resolving detector 7.

The exit angle is to be understood as the angle between the rayreflected back and the axis 15. For example, for the ray D, its exitangle 6 is drawn in.

As a result of rotation about the axis 15, the spatial, 3-dimensionalcourses of the rays are yielded. This results in the formation on thesphere surface of that annular radiation zone Z which the detector 7receives as circular annular area 8.

FIG. 4 is a camera picture of the reflected transparent sphere using thetheodolite system mentioned at the beginning. The picture shows theannularly radiating zone Z of the sphere. It is used for themathematical evaluation for determining the center of the target. Thereflection of the radiation close to the axis is also seen at thecenter.

I claim:
 1. An optical device for determining the position of areflective target (2), having a light projector (1; 30) for illuminatingthe target (2), an imaging system (5) for imaging the target (2) on alocation-resolving detector apparatus (7), and an evaluating apparatus(9) for determining position coordinates of a target image (8), whereinthe target (2) includes a ball transparent to the light projector'slight with a refractive index n between √2 and 2 such that upon itsillumination by the light projector (1; 30) and its imaging by theimaging system (5), at least a portion of a circular area (8) is formedand is detected by the detector apparatus (7) so that an imageevaluation of a center of the circular area (8) is determined and thusthe position of the target (2) is given.
 2. An optical device accordingto claim 1, wherein for the purpose of sighting the target (2) from allsides, a rod-shaped mount (4) is provided as a sphere support.
 3. Anoptical device according to claim 1, wherein the target (2) is composedof glass having a refractive index n of 1.83.
 4. An optical deviceaccording to claim 1, wherein the target (2) is surface-polished.
 5. Anoptical device according to claim 1, wherein the target (2) has areflection-enhancing coating.
 6. An optical device according to claim 1,wherein the target (2) has materials of different refractive indices. 7.An optical device according to claim 1, wherein the target is partiallysilvered.
 8. An optical device according to claim 1, wherein theillumination beam path to the target (2) runs axially symmetrically tothe optical axis (15) of the imaging system (5).
 9. An optical deviceaccording to claim 1, wherein the imaging system (5) has a focusingdevice (6).
 10. An optical device according to claim 1, wherein theimaging system (5) is a theodolite telescope.
 11. An optical deviceaccording to claim 1, wherein the detector apparatus (7) is a CCD arrayor a video camera.
 12. An optical device according to claim 1, furthercomprising an image processing computer (10).
 13. An optical deviceaccording to claim 1, wherein the light projector (1; 30), imagingsystem (5), detector apparatus (7) and evaluating apparatus (9) form anapparatus of compact structure.
 14. An optical device according to claim1, further comprising a distance measuring device (11).
 15. An opticaldevice according to claim 1, wherein a circle-fitting algorithm for thebest fitting of a circle into the area (8) is provided for the imageevaluation in order to determine the center of said at least a portionof a circular area.
 16. A method for determining the position of areflective target (2), using a light projector (1; 30) for illuminatingthe target (2), a focusable imaging system (5) for imaging the target(2) on a location-resolving detector apparatus (7), and an evaluatingapparatus (9) for determining location coordinates of a target image(8), comprising the step of using a target (2) which includes atransparent ball with a refractive index n between √2 and 2 which isilluminated with the light projector (1; 30) such that upon defocusingof the imaging system (5), multiple circle areas thus occurring areimaged on the detector apparatus (7) and at least one center of an areais determined by mathematical techniques to provide the position of thetarget (2).
 17. A reflective target for measuring, comprising:atransparent ball having a refractive index n between √2 and 2 such thatupon illumination of the ball, an at least partially circular image iscreated whose center indicates the position of the ball.
 18. Areflective target as set forth in claim 17, wherein the target iscomposed of glass having a refractive index n of 1.83.
 19. A reflectivetarget as set forth in claim 17, wherein the target is surface-polished.20. A reflective target as set forth in claim 17, wherein the target hasa reflection-enhancing coating.
 21. A reflective target as set forth inclaim 17, wherein the target has materials of different refractiveindices.
 22. A reflective target as set forth in claim 17, wherein thetarget is partially silvered.